This invention relates to a three force balanced mechanism utilized in measurements and controls in various fields of industries.
In order to automatically control production steps or the like, it is essential to measure various industrial quantities. For example, by using a three force balanced mechanism utilizing a force balancing system, such process quantities as temperature, pressure, liquid level, and flow can be measured and these measured process quantities are converted into air pressure signals, electric signals, etc. to produce converted signals. These converted signals are measured or transmittted to a pressure meter, a differential pressure meter, a controller meter or type of receivers. We have already proposed an improved three force balanced mechanism in which a floating pivot applied with these forces is provided with a cylindrical surface, one end of a span setting member is wound about the cylindrical surface and the other end of the span setting member is moved along a graduated arcuate groove, whereby the one end of the member is wound about or unwound from the cylindrical surface so as to vary the measuring range. We have filed Japanese patent application No. 170174/1982 entitled three force balanced mechanism and filed on Sept. 29, 1982.
With this improved three force balanced mechanism, the shift of zero point at the time of varying the measuring range still occurs. For example, in a pressure difference meter capable of producing an output P of 0.2 to 1.0 Kgf/cm.sup.2 as shown in FIG. 1, where the measuring range of from 0 to 5500 mm H.sub.2 O set in the output span corresponding to a characteristic shown by a solid line B is varied to a range of from 0 to 500 mm H.sub.2 O, the characteristics would become as shown by dotted lines b or c starting from zero points shifted upward or downward so that a characteristic having a zero point (0.2 kgf/cm.sup.2) as shown by a solid line A can not be obtained.
Consequently, even with a span corresponding to the desired measuring range, the correspondng for zero point outputs for respective ranges become different in that the zero point adjustment is necessary each time the measuring range is varied. Such difficulty also occurs in a prior art meter not improved as that disclosed in the three force balanced mechanism disclosed in the Japanese patent application referred to above.